[cmake] Refactor warning flag logic to use Unix warnings with clang-cl

Summary:
clang-cl understands the GCC-style -W[no-]foo flags, and for the most
part ignores MSVC -wd flags. So, let's pass the curated set of warning
flags we use on Unix on Windows. We can also stop passing /W4 -wd*,
which for the most part corresponds to -Wall -Wextra with a bunch of
flags that we mostly ignore.

I had to disable -Wnon-virtual-dtor on Windows, because it fires on
every COM class ever. I filed PR32286 to fix this.

So far I've only found two instances of -Wstring-conversion in the
WinASan code, which I'll fix. Other than that we seem clean.

Reviewers: hans

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D30992

llvm-svn: 297964